The present invention relates to a method and apparatus for storing and dispensing a liquid composed of an oxygen containing mixture, for instance a mixture of oxygen and nitrogen. More particularly, the present invention relates to such a method and apparatus in which the liquid is stored and dispensed from a container in a manner to ensure that the liquid being dispensed will contain no more than a predetermined concentration of the oxygen. Even more particularly, the present invention relates to such a method and apparatus in which dispensing is prevented when a volume of liquid remaining in the container is equal to a hypothetical volume of the liquid in a saturated state that is calculated at a particular dispensing pressure to contain the predetermined concentration of the oxygen.
The storage and dispensing of oxygen containing mixtures (for instance, synthesized mixtures of oxygen and nitrogen or liquid air for that matter) can be problematical because the nitrogen will preferentially boil off before the oxygen. The end result will be that a liquid will remain that becomes ever enriched in oxygen. Oxygen enriched mixtures can be particularly dangerous around hydrocarbons. For this reason, the prior art has provided numerous pressure relief devices in which liquid from the bottom of the container is passed through a heat exchanger in the head space of the container to collapse nitrogen enriched vapor back into the liquid. The liquid is then vaporized and vented. Examples of this can be found in U.S. Pat. No. 5,571,231 in which an external condensing coil system is provided to allow conversion of a standard liquefied gas container for use in storing mixtures of liquid oxygen and liquid nitrogen.
The shortcoming of this prior art method is that while there is no net change in bulk concentration, local variations in concentration are not guaranteed. As such, there is never a guarantee that the mixture actually being dispensed will not in fact exceed the permissible oxygen concentration.
As will be discussed, the present invention provides a method of storing and dispensing a liquid consisting of an oxygen containing mixture to prevent the dispensed liquid from having an oxygen concentration above a predetermined, allowable level.